A number of antibodies have been used in an attempt to functionally characterize distinct rabbit lymphoid cell populations, to identify cell surface markers for them and to relate these populations to those that are found in humans. These antibodies have come from a variety of sources including immunization with rabbit proteins and lymphoid cell lines, from commercial sources and from other investigators in this field. Various techniques for peripheral blood cell isolation have been tried in order to obtain preparations of peripheral blood cells that are uniform with respect to distribution of cell type and also with respect to their response to stimulation with various T- and B-cell mitogens. An antibody, 93C6, that reacts with activated T cells has been identified and its antigen has been found on spleen and appendix cells but not on thymus. Thymus cells become positive when stimulated with a T cell mitogen. Preliminary, results suggest that the antigen of 93C6 may be a component chain of the interleukin-2 receptor. A second monoclonal antibody, M110, was found to react with a small subset of cells from peripheral blood in the rabbit and with certain rabbit T cell lines prepared by transformation with HTLV-1. The antigen recognized by M110, is currently being sought and its cell distribution is being examined in various lymphoid organs of the rabbit. Attempts to characterize a CD4 homolog to serve as a marker for the T helper subset of rabbit lymphocytes has not thus far been successful. A number of attempts at both the protein and nucleic acid level have not yielded a likely candidate. Current attempts include the use of various PCR primers based on regions of the CD4 sequences conserved among the human, rat and mouse molecules.